Apparatus for molding cellular plastic articles



Dec. 19, 1961 c. A. RICHIE ETAL APPARATUS FOR MOLDING CELLULAR PLASTIC ARTICLES Filed Nov. '7, 1960 t' f lllllllIIIIIIIIIII- I WW? INV EN TORS CARLTON A. RICHIE A TTORNE X5 m m w will!!! FIGZ FIGS

ANTHONY J. SCALORA w. (2. $1M W m $5M FIGAI ii g {a A r e kg w, si rra l a a a t d s! 3.15.1 with Q r Pat t d D 19 i 2 3 m3 M areas from which it follows that the more dense areas 5 w will be stronger than the less dense areas. Moreover, the APPARATUS FQR gELLEJLAR PEASTW present invention relates to a method of making the store Carlton A. Richie and Anthony .i. @calora, Toledo, Ohio,

assignors to Owens-Illinois Glass Company, a corporation of Ghio Filed Nov. '7, 196i), Ser. No. 67,713 7 Gaines. (Cl. 18- 12) This invention relates to an apparatus for molding cel lular plastic articles. In one aspect this invention relates to an apparatus for making such articles by using a com pressed gas to eliect a differential distribution of foamable or expandable cellular plastic beads from which the articles are made.

in the manufacture of cellular plastic articles from expandable cellular plastic beads the problem has arisen as to just how these plastic beads might be most effectively distributed in the mold cavity during the molding of articles from such beads. For the molding of some articles, it would be desirable to distribute the beads in a substantially uniform manner throughout the mold cavity. On the other hand in the manufacture of certain containers it would be desirable to form the same with walls which have variable densities in order that the container will be stronger in an area which has to withstand the greatest stress and strain and less strong in an area which is subject to less stress and strain or by its geometry is better reinforced to withstand stress and strain. This has the further advantage in that it results in a more efficient utilization of a given amount of molding material used to make the container. Also, such diiierential density of the container walls is contrary to the customary molding practiced which is to form any molded article with walls of uniform density so as to avoid so-called thin areas.

It is, therefore, an important object of this invention to provide an apparatus for distributing particulate expandable plastic materials throughout the mold cavity before final expansion and molding of the plastic.

Another object of this invention is to provide an apparatus for making cellular plastic articles, and particularly containers, whose walls are stronger in certain desired areas than in other areas.

Another object of this invention is to provide an apparatus for making containers whose walls are or" variable density.

Another object of this invention is to provide an apparatus for fabricating the walls of containers whose walls will be adapted to withstand variable stresses and strains.

An additional object is to provide an apparatus for shaping containers from cellular plastic materials, such as polystyrene.

A further object of this invention is to provide an apparatus for shaping plastic containers in which the walls are more dense in those areas of greatest stress and shock and less dense in those areas which are subject to lesser stress and shock. v

A still further object is to provide an apparatus for producing cellular plastic articles in which the plastic materials which make up the articles are more efliciently utilized to provide articles of maximum strength under varying conditions of stress, strain, and shock.

In general the present invention relates to a new apparatus for producing cellular plastic articles from cellular plastic material by distributing said material by a gaseous medium under pressure. A more specific form of this invention contemplates the manufacture of cellular plastic articles whose side walls are made of a cellular plastic material which is difierentially distributed so as to provide areas which are more dense than other mention'ed articles in which a flow of gas-is used to effect the differential distribution of the cellular plastic materials or partially expanded plastic beads in a mold cavity during the portion of the molding cycle during which the mold is closing, and after the mold is closed and the flow of gas discontinued, heating the male and/or female mold member so as to effect a further expansion of the beads so as to fill the interstices between the beads and cause the mold charge to conform to the shape of the mold. The

' male mold member can then be retracted and the formed article removed from the mold cavity.

Further objects and advantages of the present invention will be apparent from the following specification and claims which include preferred embodiments of the present invention, and from the drawings, hereby made a part of the specification, wherein:

FIGURE 1 is a schematic cross section view showing the mold elements in the open position;

FIGURE 2 is a schematic cross sectional view showing the mold elements in a partially closed position;

FIGURE 3 is a schematic cross sectional view showing the mold elements in a fully closed position, and

FIGURE 4 is a sectional view of a container made in accordance with the method of this invention.

Referring to the drawings, the plastic container X is illustrative of articles made by the novel apparatus of this invention. Container X shown in FIGURE 4 consists of a bottom wall a and side walls 0 and c of uniform thickness. It will be noted that not only are the side walls c and c more dense than bottom wall a, but that the upper areas of side walls 0 and c are more dense than the lower areas. This is desirable where container X is used as a drinking cup since the greatest density and therefore the greatest strength is located where the cup is subjected to the greatest strain, that is, lateral compression forces resulting from gripping of the cup by the user thereof. It will also be evident that a container such as that described above will for a given amount of plastic material provide the most efiicient utilization of the plastic. Moreover, it will be noted that if the container walls were of uniform density, the strength thereof would} be the same throughout such that certain areas of the container walls which are subject to lesser amounts of shock would be stronger than necessary while areas which need to withstand greater stresses would be of no greater density or strength and accordingly would be more prone to breakage in those areas.

The relationship of density of plastic to strength is well illustrated by the following table: 1 TABLE I Correlation of strength with density It will be apparent from the table above that if container X of FIGURE 4 has a 2 lb. .per cu. ft. density in bottom wall a, a 4 lb. per cu. ft. density the lower por- I densities may be employed depending upon the weight of plastic charge originally placed into the mold cavity and the degree of strength desired of the container being formed. Accordingly densities ranging from 1 lb. per cu. ft. to 10 lbs. per cu. ft. may be employed.

A preferred apparatus for making the article of FIG- URE 4 and suitable apparatus for accomplishing the same is set forth below. More specifically and with reference to FIGURES 1, 2 and 3 there is shown a molding apparatus in open position having a male die member or force plug 1 which is integral with a plunger 2 which has conventional means, not shown, for reciprocating the same. The force plug 1 which is drilled for circulating steam and water, is mounted inside of a spring loaded sleeve 3. Instead of a drilled force plug a porous metal plug can be employed, if desired. An air inlet 4 connected to a suitable source of air under pressure runs down through the force plug 1 terminating in an opening 5 in the bottom of said plug 1. The lower mold 7 is provided with steam heating coils 8 through which water can also be circulated for cooling purposes.

In operation a plastic bead charge P, such as partially expanded polystyrene, is placed in the mold cavity 6. One method of effecting this introduction of the bead charge into the mold cavity would be by the well known injection means, not shown, as taught by either U.S. Patent 2,155,316 or U.S. Patent 2,235,222, or by hand or any other suitable means. Pressure is applied to the plunger 2 which results in the downward movement of force plug 1 whereby sleeve 3 mounted on springs 9 contacts the upper surface 11 of mold cavity 7 as shown in FIGURE 2. When the sleeve 3 contacts the upper surface 10, air from the compressed air source is caused to enter inlet 4 and flow upwardly and continuously during the downward movement of the force plug through the mold cavity and out through grooves 11 which are on the lower surface of sleeve 3. FIGURE 2 is a view shortly after sleeve 3 has contacted surface and shows the springs 9 beginning to be compressed. In this particular embodiment the flow of air is maintained at a high enough rate to distribute the beads throughout the mold cavity and to cause the beads in the upper portion of the mold cavity to be more closely compacted than in the lower portion. The force plug 1 then continues to its downmost position while air continuously flows through the mold cavity, as shown by FIGURE 3, at which time air flow is discontinued and steam is caused to enter the heating coils 8 causing the beads to expand and coalesce to form the finished article X. The heating with steam is continued for a time sufficient to give the heads the requisite expansion and to cause them to adhere into a coherent mass. Thereafter a coolant is passed through these coils to discontinue expansion of the beads and set up the article. The force plug 1 is then withdrawn from the mold cavity 6 and the article X removed. The resulting article X as shown in FIGURE 4 will be found unitary in structure and have a greater strength in its two vertical side members, c and c than in its bottom member a. This is particularly desirable in the case of drinking cups made by this process which are subject to lateral compressive forces.

Although the foregoing represents a preferred method of utilizing the novel apparatus of this invention particularly as it relates to the manufacture of cellular plastic articles having areas of differing densities, the apparatus can also be used for the more general process, namely, the distribution of plastic materials and especially plastic beads by continuously flowing a gas through a mold cavity during the molding operation so as to distribute the beads either in a uniform or differential manner throughout the mold cavity, as desired. Where a substantially uniform distribution of the plastic beads occurs, the article formed will, of course, have a substantially uniform density in contrast to the article of FIGURE 4.

A particularly suitable polystyrene plastic is that designated as Dylite which is in the form of beads and made by the Koppers Chemical Company. These bonds can be given a pro-expansion treatment which consists of heating the original or virgin beads at 180-240" F. until a density of 1-5 lbs. per cu. ft. is obtained. These poly styrene beads contain a volatile liquid, n-pentane, as an expanding agent. However, particulate polystyrene containing any volatile liquid expanding agent can he employed. Moreover, particulate polyst renc containing one of the solid expansion agents which decompose on heating to yield a gaseous expansion agent can also be used. Although polystyrene is preferred, any plastic can be used which is capable of being formed and further expanded upon the application of additional heat in the manner described above.

The temperatures required to expand and fuse the cellular or pro-expanded polystyrene beads in the mold ranges from 180 to 240 F. After completion of the molding cycle which takes on the order of 5-15 seconds, the force plug and mold cavity are cooled to at least F. at which temperature the formed article can be stripped readily from the force plug.

The air pressures usually employed are from 3-10 lbs. p.s.i. with a pressure of 3-5 lbs. p.s'.i. being the preferred range.

In using the apparatus here disclosed it is important that the compressed air inlet be positioned where minimum density of plastic particles is desired since it has been observed that the distribution of the particles is densest in the extremities of the mold cavity (upper regions of areas 0 and c of FIGURE 2) or those areas most distant from the compressed air inlet. Since the mold cavity is only partially filled with the plastic initially, as shown in FIGURE 1, the upper portions of c and c, which in the finished article are subject to the most stress due to lateral strain, are positioned farthest from the inlet 4. When the air under pressure which is moving at a relatively high rate of fiow is introduced through the inlet 4, most of the beads are distributed to the upper portions of c and c and the density of the heads is greater here than in area a and the lower portions of areas c and c. Accordingly the strength of the upper portions of areas c and c in the finished article is greater than in area a and the lower portions of areas c and 0. However, one can use a low pressure or a relatively low rate of flow of air in the side areas for certain purposes. For example, such an article would be desirable where the bottom thereof is subject to greater mechanical shock than the sides as in the case where the article was likely to be roughly handled by being frequently dropped or caused to fall on its lower or bottom side. Under such circumstances the article may not be subject to serious lateral forces to its sides and accordingly less dense sides would be suitable. From the foregoing explanation it will be evident that the position of the air inlets and the velocity of the incoming compressed air will determine the manner in which the plastic beads will be differentially distributed throughout the mold cavity 6.

It is understood that the invention as described above is a preferred embodiment and that certain variations can be made without affecting the basic operation of the apparatus disclosed. For example, the mold cavity can have other shapes than that shown. Moreover, the steam coil heating means can be replaced by electrical heating means such as induction and resistance heating if deemed desirable. Other conventional heating means can be employed including direct steam injection wherein, for example, the inside walls of lower mold member 7 or forceplug 1 or both are perforated and connected to a source of steam. In addition to facilitate removal of the finished article, the force plug and/or the lower mold cavity can be coated with any of the well known mold lubricants such as the silicones. It is also apparent to one skilled in the art that the improvement in molding apparatus. disclosed by this invention is readily adaptable to molding machines employing a plurality of force plugs and mold cavities as typified by US. Patents 2,115,316,- 2,183,869 and 2,235,222. Moreover, the air opening is shown as being located in the force plug but it is within the purview of this invention that an air opening can be located in the lower mold cavity, immediately opposite and below opening 5. However, any such opening for the incoming air or gas under pressure should be so placed on either the force plug 1 or lower mold 7 so as to cite-ct preferably an upward flow of the gas throughout the entire cavity formed by plug 1 and mold 7 during closing of the mold (FIG. 2). Of course, it will be evident that in the closing of the mold (FIG. 2) to its final position (FIG. 3), plug 1 and mold 7 will also assist in distributing the plastic bead particles P. Thus, it will be noted that the cross sectional area or" the annular space between the force plug 1 and cavity 6 is larger in FIG. 2 than in the final molding position of FIG. 3. Therefore, it is easier, to blow the plastic into this larger space; more importantly the plastic is slightly compressed as force plug 1 moves downwardly and is thereby held in place by bridging so that the beads stay in place even after the gas flow is discontinued as in FIGURE 3, which shows the mold in the final and completely closed position. In addition any heat expansible particulate plastic can be employed in the process. It will also be evident that a gas other than air can be used to effect distribution of the plastic particles. Preferably the gas should be unreactive with the plastic beads and accordingly gases such as nitrogen, argon, carbon dioxide, steam and the like can be effectively employed. Furthermore, both the force plug and mold cavity can be provided with heating means. Other appropriate changes can be made within the skill of one familiar with the art.

From the foregoing description it will be apparent that we have devised a novel plastic molding apparatus in which compressed air is not usedto shape directly plastic sheet material into the desired article, such as a plastic cup, as is well known in the art but to efiect a differential distribution of the molding materials prior to form shaping by the mold members. This technique is particularly significant with respect to molding partially expanded thermoplastic beads, such as polystyrene, which are found to be admirably adapted for manufacturing lightweight low heat transfer plastic containers in accordance with the apparatus disclosed by this invention because of their excellent insulating properties and their remarkable storage life at very low temperatures.

Representative embodiments of the invention have been described above, but it is to be understood that these are for illustrative purposes only, and that the invention herein disclosed may take various other forms coming within the scope of the appended claims.

This application is a continuation-in-part of Serial No. 841,342 filed September 21, 1959, and now abandoned.

What we claim is:

1. In an apparatus for molding foamable plastic beads, the improvement comprising a lower mold member having a mold cavity, an upper mold member supported with respect to said lower mold member, for cooperation therewith, one of said mold members having a passage for supplying a bead distributing gas upwardly under pres- 2. In an apparatus for molding foamable plastic beads, 7 the improvement comprising a lower mold member having a mold cavity, an upper mold'member supported with respect to said lower mold member for cooperation therewith, one of said mold members having a passage for supplying a bead distributing gas upwardly under pressure to said mold cavity when said upper mold member enters said lower mold member, means for heating the molding apparatus, a spring mounted sleeve on said upper mold member, and said sleeve having grooves which define an exhaust opening when said sleeve contacts such lower mold member.

3. The apparatus of claim 1 in which the passage for supplying the gas under pressure is located in the upper mold member.

4. The apparatus of claim 1 in which the passage for supplying the gas under pressure is located in the lower mold member.

5. An apparatus for molding particulate plastic material comprising a lower mold member having a mold cavity, an upper mold member supported with respect to said lower mold member for coaction therewith to form a closed mold which has an initial closed position and a final closed molding position, said upper mold member including a mold plug extending substantially into said 7 lower mold cavity to define an annular space between said lower mold member and said plug, means 'for advancing said plug into said cavity to a final closed molding position, gas inlet means for introducing a gaseous fluid into the bottom portion of said mold cavity for the purpose of distributing the particulate material therein by direct contact with said fluid and gas outlet means for exhausting said gas from the upper portion of said annular space during the time thatsaid plug is moving downwardly from said initial closed position to said final position, said annular space having a greater volume in said initial closed position than in said final closed molding position.

6. In an apparatus for molding foamable plastic beads, the improvement comprising a first mold member having a mold cavity, a second mold member supported with respect to said first mold member, for cooperation therewith, one of said mold members having a passage for supplying a bead distributing gas under pressure to said mold cavity when said second mold member enters said first mold member, means for heating the molding apparatus, and a spring mounted sleeve on said second mold member.

7. In an apparatus for molding foamable plastic heads, the improvement comprising a first mold member having a mold cavity, a second mold member supported with respect to said first mold member for cooperation therewith, one of said mold. members having a passage for supplying a bead distributing gas under pressure to said 'mold cavity when said second mold member enters said first mold member, means for heating the molding apparatus, a spring mounted sleeve on said second mold memher, and said sleeve having grooves which define an exhaust opening when said sleeve contacts such first mold member.

References Cited in the file of this patent UNITED STATES PATENTS 

